The proposed research is directed towards understanding the molecular mechanisms regulating gene expression in a simple eucaryotic organism, the yeast Saccharomyces cerevisiae. In particular, we propose to continue our studies of two aspects: (1) The mechanism by which the mating type locus determines yeast cell type. (2) The mechanism by which the two yeast cell types are interconverted. We shall continue or studies on the structure of the mating type locus by performing additional complementation and mapping tests. We shall characterize newly-isolated mutants defective in mating to determine whether they have defects in agglutination or pheromone synthesis. In addition, we shall continue our studies on the pheromones of S. Kluyveri in order to identify the structural gene for alpha-factor and genes involved in response to alpha-factor. We have proposed that yeast cell type is determined by genetic rearrangement. The "cassette hypothesis" states that yeast cells contain silent copies of a and alpha mating type loci which become activated by insertion into the mating type locus. We propose further that genes HMa and HMalpha are the silent copies of a and alpha information. This hypothesis shall be tested by isolation and characterization of mutations affecting the interconversion process. BIBLIOGRAPHIC REFERENCES: Hicks, J.B. and I. Herskowitz (1977). Interconversion of yeast mating types. II. Restoration of mating ability to sterile mutants in homothallic and heterothallic strains. Genetics 87: in press. Hicks, J.B., J.N. Strathern, and I. Herskowitz (1977). Interconversion of yeast mating types. III. Action of the homothallism (HO) gene in cells homozygous for the mating type locus. Genetics 87: in press.